1. Field of the Invention
The present invention relates to a semiconductor device and more specifically to a gate turn-off thyristor of buried gate structure type, which can increase the controllable current (a current flowing between the anode and the cathode) and make uniform the turned-on voltage (a voltage drop between the anode and the cathode).
2. Description of the Prior Art
In recent years, gate turn-off thyristors (referred to as GTO hereinafter) have widely been used in place of ordinary thyristors. This is because the GTOs can be turned off without use of any turning-off circuits including at least one inductor. Additionally, the GTO is high in efficiency and small in size, and further can control a high voltage and a large current as an excellent switching element. The above GTO can roughly be classified into two groups of surface gate type and buried gate type on the basis of the gate structure. In the surface gate GTO, since plural complicatedly-shaped gate electrodes are formed being exposed on the device surface, it is rather difficult to reliably realize a minitualized gate structure in dependence upon the present manufacturing technique. In contrast with this, in the buried gate GTO, since a high-concentration diffusion layer used as gate is formed being buried in a semiconductor layer and further covered by another expitaxially-formed layer (called buried gate), it is relatively easy to reliably realize a minitualized gate structure. The reason why the miniatulized gate structure is required is as follows: when the impurity atom concentration in the expitaxially formed layer is reduced, since it is possible to markedly increase the break-down voltage at the gate-cathode junction, it is possible to improve the turn-off characteristics of the GTO by applying a relatively high reverse voltage between the gate and the cathode. To further improve the turn-off characteristics, it is preferable to minitualize the gate structure for allowing the gate current to uniformly flow through the low-resistance gate layer.
In the buried gate GTO, it is further well known that when the impulity atom concentration of the diffusion layer is increased but the whole electric gate sheet resistance (determined by the total amount of dope) is reduced to decrease the resistance of the buried gate, it is possible to obtain a large gate current, that is, to improve the turn-off characteristics of the GTO. However, in the case where the electric gate resistance is excessively reduced, there exists a problem in that crystal defects readily occur in accordance with the high concentration diffusion layer. Since these crystal defects shorten the lifetime of the minority carriers in the N-type and the P-type semiconductor layers, there exists a problem in that the GTO characteristics inevitably disperse within the device and/or among the devices.